The invention relates to spatial error corrector circuits, and particularly to an analog error corrector for independently correcting geometrical scan errors in the four corners of camera tube rasters.
Typical prior art systems for the analog correction of geometrical scan errors, generally provide means for generating a fixed, full-field waveform (horizontal sawtooth modulated by vertical sawtooth and/or horizontal parabola modulated by vertical parabola) by any of several means, and for applying a portion of it to each channel. This gives rise to corrections generally referred to as "keystone" and "pincushion", respectively. The two major shortcomings of this method are; first, since the correction waveform is fixed for the full field, a correction applied to one corner of the raster will of necessity affect the other three corners a like amount in a different direction, making adjustment very difficult; second, inherently, this method can only correct symmetrical (or antisymmetrical) errors, whereas actual errors are generally non-symmetrical. Attempts have been made to provide a circuit which generates such a full-field waveform and subsequently separates the four corner quadrant signals by a gating circuit. The circuit produces the desired corner-to-corner independence but unavoidable transients and offsets in the gating process seriously degraded the resultant picture and thus such a circuit was not developed.